Hermetically sealed interfaces have been fabricated by the application of metal and glass structures. Metal and glass are often used to create these hermetically sealed interfaces because of the need to have a conductive element, such as metal, for carrying electrical signals across a hermetically sealed interface typically constructed with a non-conductive element, such as glass. These materials are fused into a structure by many different processes. These processes can include the disposing of a liquefied metal into, a solid glass structure having a passageway therein, and then the solidification of the metal forming a hermetically sealed interface.
All of these, metal and glass, structures can suffer from a common problem. This problem is the failure of the fused hermetic interface when the metal and glass structures are subjected to a large temperature transition. These failures occur because of the difference in the thermal coefficient of expansion of metal and glass. For example, if the metal is solder, the solder will shrink to a much greater extent than the glass during a falling temperature transition. This results in a large strain at the fused hermetic interface, causing a crack in the glass, a crack directly between the solder and the glass, or a crack in the solder. Many thermal transitions may be necessary before these failures of the hermetic interface occur.
In a specific example, FIG. 1 illustrates a cross section of a prior art silicon capacitive pressure sensor using a metal and glass structure to hermetically seal an interface between a pressurized sealed chamber 125, in which a reference pressure is stored, and the top of a glass substrate 101, where electrical interconnection to the capacitive element is provided. The glass substrate 101 has a first surface 103 and an opposing second surface 105. A first passageway 107, and a second passageway 109 are provided through the glass substrate 101. The first passageway 107 is then processed to include an electrically conductive metal layer 111 that extends to a first predetermined area 113 on the opposing second surface 105. The second passageway 109 is also processed to include a metal layer 115 that extends to a second predetermined area 117 on the opposing second surface 105 of the glass substrate 101.
A semiconductor substrate 119 is then attached to the opposing second surface 105 of the glass substrate 101 at locations shown by reference number 121 and to the second predetermined area 117. This attachment between the glass on the opposing second surface 105 and the semiconductor substrate 119, at locations shown by reference number 121, is performed by anodic, or electrostatic, bonding. This anodic bond hermetically seals the glass and semiconductor substrate at locations shown by reference number 121. The bond between the metal layer 115 that extends to a second predetermined area 117 and the semiconductor substrate 119, indicated by reference number 129, is the result of a chemical reaction that fuses the semiconductor substrate 119 and the metal layer 115 with heat and pressure.
The chamber 125 is then pressurized and a quantity of solder 123, 127 is melted into the first and second passageways 107, 109. The solder 123 is the first passageway, when cool, forms a hermetic interface for sealing the chamber 125 and provides an electrical contact to an element of a capacitor, formed by the metal layer covering the predetermined area 113. The solder 127 in the second passageway 109 provides an electrical connection to a second element of the capacitor formed by the semiconductor substrate 119. The solder 123, 127 formed in the first and second passageways 107, 109 is used to connect the capacitor to a measurement circuit. When this capacitor is subjected to differing pressures, a portion of the semiconductor substrate 119 moves, in relationship to the metalized first predetermined area 113, causing a change in distance between the elements of the capacitor and thus capacitance.
The construction and hermetic sealing of the first passageway 107 is of particular concern. The temperature coefficient of solder and glass is substantially different. When the solder and glass structures are subjected to a temperature transition they expand or contract at different rates because of differing thermal coefficients of expansion. The differing rates cause stress to build up in the solder, 123 and at the location of the glass 102 and solder 123 interface. This is undesirable because during large temperature transitions the glass 102 and solder 123 will be over stressed and either or both will crack. This cracking causes the hermetic seal to be broken. When this seal is broken the reference pressure is released and the sensor no longer can function as desgned. Similar cracks in the glass-solder interface for the second passageway 109 have no effect on the chamber 125 because of the anodic bond at the location shown by reference number 121. This anodic bond isolates the chamber 125 from any breaches in the second passagway 109.
What is needed is an improved way of hermetically sealing this first passageway while maintaining an electrically conductive path between the surfaces 103 and 105.